Pressure process for impregnating wood



Feb. 9, 1954 M. J. HERMAN PRESSURE PROCESS FOR IMPREGNATING WOOD 2Sheets-Sheet 1 Filed Deo. 16, 1948 I FJ Woe/f TANK TREAT/NG CyL/NoneMensa/ef ,0a/np ADJUJTABLE naw @Nr/@OL TIME //V MINUTES Mae w/v u'.Alge/wmv Feb. 9, 1954 M. J. HERMAN PRESSURE PROCESS FOR IMPREGNATINGWOOD Filed Dec' le' 1948 2 sheets-sheenw 2 N Q s u) "I un I y.. w

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marre/2Min Patented Feb. 9, `1954 Mervin J. Herman,` rrville, Ohio,`assigner to Koppers. Company Inc., Pittsburgh,- Pa., a corporation of.Delaware Application December 16, 1948, Serial No. 65,663

7 Glaims. (Cl. 117-59) This invention relates to wood preservation, and?particularly toA an improved pressure process or method for impregnatingwood with preservative material.

In order to prevent decay of wood or timbers, and thereby to increasetheir life, itis common practice to impregnate the wood or timbers witha preservative, such as creosote or other material. The protectionafforded by the application of preservative material is dependent upondeep and reasonably uniform penetration of the Wood or timber by thepreservative material. Itis customary to treat wood or timbers withpreservativefin closed cylinders under pressure so the preservativematerial is forced into the body of the wood.

However, with known methods of impregnation, it has been found that itis very difficult, if not impossible, when treating some types of greenor incompletely seasoned wood, such as soft pine or other conifers, tosecure adequate and uniform penetration of the preservative. This isparticularly true when the Wood which is being treated is green or isnot fully seasoned, and is of a type having a louT specific gravity,such as less than 0.45.

In the processes now employed wood is subjected to preservative underrelatively high pressure, such as 150 to 200 pounds to the square inch,for a substantial period of time, such as 4 or 5 hours, and at arelatively high temperature. such as 180 F'. to 210 F. In some cases,before the wood is impregnated, it is subjected to a preliminarytreatment with steam under W pressure, such as 2O pounds to the squareinch. Al-

though increases in pressure. tend to increase the `amount ofpreservative absorbed by the Wood, it may cause the penetration to beerratic or uneven, that is, to cause the depth to which the preservativepenetrates to be much less in some areas than in others. It appears thatthe application of pressure causes compression of the outer layers ofthe Wood, particularly after the Wood is weakened and softened bysteaming. The compression of the outer layers of the Wood causes`collapse of the cells of the wood. This collapse of the Wood cellsoccurs Where high pressure is applied While the cells are empty andbefore they have become lled with preservative, This co1- lapse. of thewood cells is especially likely to occur the wood cells in an area,there is formed a relativelygmpenetrablelayer which restricts, or even 2the interior of the Wood, and thus makes it impossible to` secure theuniform, deep penetration of the preservativeV which is essential tosatisfactory preservation of the Wood.

An object of this invention is to provide an improved method or processof pressure-treating wood.

A further object of the invention is to provide an improved process ormethod of the type described which makes it possible to successfullytreat wood or timbers which cannot 'be satisfactorily treated by themethods heretofore known.

Another object of the invention is to provide an improved process ormethod of pressure-treating Wood in which the preservative material issupplied to the pressure vessel at a rate governed by the abilityy ofthe wood being treated to absorb the preservative.

A further object of the invention is to provide an improved process ormethod of pressure-treating wood which insures that uniform deeppenetration of the Wood being treated will always be secured.

Another object of the invention is to provide an improved process ormethod of pressure-treating Wood which operates to control the pressureto which the wood is subjected to values which Will insure rapidabsorption of the preservative, but will not` cause collapse of thecells of the Wood.

A further object of the invention is to provide an improved process ormethod of the type described Which automaticallyv operates to restrictthe pressure to which the Wood is subjected to relatively low valuesuntil the preservative has penetrated the Wood to a substantial depth sothat the cells in the outer layers of the Wood become filled withpreservative and do not collapse When the pressure is increased.

Another object of the invention is to provide an improved process ormethod of pressure-treating wood which may be employed regardless orwhether the wood is being treated by the fullcell process, the Lowryprocess, or the Rueping process.

A further object of the invention is to provide an improved process ormethod of the type described` Which can be carried out automaticallyand` which requires a minimum` of attention.

Another object of the invention is to provide an improved woodpreserving process which is arranged so,- that. the pressure to whichthe wood is subjected is gradually increased in accordance withincreases inl thcpressure gradient from the suriace of thewood. totheuntreated areas within the wood as the depth of penetration of thepreservative liquid increases.

Other objects of the invention and features of novelty will be apparentfrom the following description taken in connection with the accompanyingdrawings.

ln the drawings, Fig. 1 is a iiow sheet illustrating diagrammaticallyapparatus suitable for carrying out my improved process; and

Figs. 2 and 3 are diagrams showing the pressure conditions andpreservative absorption throughout a treating cycle under my improvedprocess, and also showing the absorption of preservative liquid by thewood being treated.

Referring to the drawings, Figure. 1 is a diagram of a wood preservingplant arranged to supply of liquid exceeds this rate, the pressure inthe treating cylinder will build up too rapidly and will collapse thecells of the wood, thereby `interfering with penetration of thepreservative into the wood. If the rate of supply of preservative liquidis less than the rate at which the employ my improved process. As shown,there is a pressure or treating cylinder in which the wood being treatedis subjected to preservative liquid under pressure,l al work tankadapted to hold a quantity of preservative liquid, and a pressure pumpfor supplying liquid under pressure from the Work tank to the treatingcylinder through an adjustable flow control means for controlling theflow of preservative into the treating cylinder.

The operation of the equipment during treat- -ment of wood by theRueping process will be described, but as explained above, the processprovided by this invention is equally applicable to the treatment ofwood by the full-cell process, or by the Lowry process.

A quantity of Wood to be treated is placed in the treating cylinder, butbefore being placed in the treating cylinder the wood is measured so.that the number of cubic feet of wood in the cylinder is accuratelyknown. In addition, the species of the wood being treated is known.After the wood has been placed in the treating cylinder, the treatingcylinder doors are sealed, and the wood may be then subjected to anydesired preliminary treatment, such as steaming. Thereafter, a vacuum isdrawn on the charge in the treating cylinder and is maintained for asuitable period, such as from one and a half to three hours. Green orpartly seasoned wood is thus treated to drive out moisture and to openthe pores of the wood.

Air is now injected into the cylinder until the desired pressure isobtained, and for purposes of illustration it will be assumed that thepressure in the cylinder is increased to pounds to the square inch Whichis the standard specication for the Rueping process. The cylinder is nowfilled with hot preservative oil in such away that the injected air istrapped in the wood. This may be accomplished by forcing'thepreservative oil into the bottom of the cylinder and allowing air toescape from the top of the cylinder just rapidly enough to maintain aconstant pressure.

After the treating cylinder has been filled with preservative, thefurther supply of liquid to the treating cylinder is regulated by thecontrol equipment shown in the drawings. This equipment is arranged tosupply to the treating cylinder a substantially constant volume ofliquid per minute irrespective of pressure conditions in the treatingcylinder.` As hereinafter pointed out, the supply of liquid to-the'treating cylinder at a uniform rate causes the pressure in thetreating cylinder to gradually inerease. The rate 0f supply of liquid tothe treating cylinder is selected inaccordance with' the species of thewood being treated and'in a'c'cordanc'sewith'jthe total volume of woodinthe treating'r 'cylinder s'th'at preservative can be absorbed by thewood, the time required for the treatment of a charge will beunnecessarily prolonged and the expense of the treatment will beunnecessarily increased.

As the volume of the wood in the charge, ,the species of the Wood, andthe rate at which 'liquid can be absorbed by this species of wood areknown, the rate of supply of liquid to the treating cylinder to produceuniform, deep penetration of the preservative in the minimum time may bereadily calculated.

As previously explained, the rate of supply of liquid to the treatingcylinder is selected in accordance with the ability of the wood beingtreated to absorb the liquid, and is substantially as rapid as possibleWithout causing collapse of the cells of the Wood. This results in agradual increase in the pressure in the treating cylinder during theinitial part of the treating'period, while this is followed by a rapidincrease in the pressure in the treating cylinder during the latter partof the treating period.

When the wood is first subjected to the pressure of the preservativeliquid in the treating cylinder, the cells of the Wood are empty, exceptfor the air under pressure present therein,l and if excessive pressureis applied to the wood, the cells will collapse and will interfere withsubsequent penetration of liquid into the wood. The liquid graduallypenetrates the cells of the wood and lls them with liquid, displacing orcompressing the air present therein, so that the pressure of the liquidin the cells builds up substantially to the pressure of the liquid inthe cylinder. After a cell has become lled with liquid, or with liquidtogether with air under pressure, it will not be collapsed as a resultof an increase in the pressure of the liquid in the treating cylinder.This is particularly true as the pressure of the liquid and air withinthe cells builds up so that the pressure on opposite sides of the wallsof the cells is substantially equal.

The cells in the outer layers of the wood be come filled with liquid orwith liquid and air under pressure first, and thereafter the liquidpenetrates inwardly to the inner layers of woodso that there is agradual increase in the thickness of the layer in which the wood cellsare filled with liquid or with liquid and air under pressure. The layersof wood present some resistance to the flow of liquid so the effectivepressure of the liquid after it reaches the inner layers of the wood ismaterially less than the pressure of the liquid in the treating cylinderat the surface of the wood. As-the layer of cells filled with liquidincreases in thickness, there is a corresponding increase in the amountof reduction in the eiectivefpres; sure ofth'e liquid applied totheempty. orpartly `ber in the charge.

:aecomo empty cells, located inwardly of vthe filled cells. Hence, eventhough the pressure of the liquid in the treating cylinder builds up toa .relatively high `value during the latter part of the treating period,the effective pressure applied to `the `Wood cells which have notalready become 4filled with liquid, that is, to the cells located asubstantial `distance below the surface of the wood, is vrelatively lovvand is below the value which will lcause collapse of these cells. Thecells which are located nearer the surface of the wood, and thereforeare subjected to higher effective pressures, become filled orsubstantially lilled with liquid before the high effective pressures aredeveloped `so these cells are not collapsed by these high pres- STATES..i

As the rate of supply yci. liquid to the treatin cylinder is selected inaccordance with the 4ability of the wood to absorb the liquid, thepressure in the treating cylinder increases only at Aa rate sufficientto cause the liquid to penetrate the Wood and be absorbed, that is tolill the cells of the wood, and does not increase so rapidly as tocause` collapse of the cells of the Wood. As collapse of the wood cellsdoes not occur, the preservative liquid can readily penetrate all areasof the wood so uniform deep penertration of the Wood by the preservativeis secured. Furthermore, as the preservative liquid is supplied at arate selected in accordance with the ability of the Wood to absorb theliquid, the processing of a charge of wood is carried out in the minimumtime, thus limiting the expense of the treatment and making possible thetreatment of a maximum amount of Wood with a set of equipment.

After the charge has been in the cylinder for a period `of time longenough to obtain the desired absorption, the further supply ofliquid tothe treating cylinder is cut off, the pressure in the cylinder isreleased, the preservative "is drained from the cylinder, and the chargeis subjected to a high final vacuum for a period of thirty minutes ormore. As soon as the pressure is released, the compressed air in theWood expands and forces out a considerable amount of the preservativethat was injected. The iinal vacuum hastens the recovery of preservativeand shortens the period during which the preservative will continue todrip from the timber. After the final vacuum has been discontinued, thecylinder door is opened and the charge is removed.

Figs. 2 and 3 are diagrams showing the pressure conditions in thecylinder during the treatment of a charge of 1200 cubic feet of Norwayor red pine by the Rueping process. Fig. 3 plots the pressure curves ofFig. 2 on a logarithmic scale in which the logarithm of the pressure isgiven as a function of tlietime. An initial air pressure of 20 poundsper square inch was established on the charge and this pressure wasmaintained while the cylinder was lled with preservative liquid.Thereafter, preservative liquid was supplied to the treating cylinder ata rate such as to cause 20,000 pounds to be supplied in 6 hours. This is1,6% pounds for each cubic foot of timber in the charge, and is at therate of 2% pounds o preservative per hour for each cubicfoot of tim- Thepressure treatment was terminated after about ilve hours and fortyminutes, whereupon the pressure was released and the charge wassubjected to a Iinal vacuum.

The broken line Illil in Fig. 2 indicates the quantity of preservativesupplied to the cylinder and it will be seen that between.l9,000 and20,000 pounds of preservative had been supplied to the 6 treating'`cylinder at the time the pressure was released. `'lhis liquid is inaddition to that required to initially ll the treating cylinder. As aresult of release of the .pressure and thev establishment `of a vacuumin the treating cylinder, considerable liquid was expelled from thetimber forming the charge and the net retained liquid totalled 9,000pounds, giving ahet retention of '7l/2 pounds per cubic foot of timberin the charge.`

The solid line loi in Figs. 2 and 3 indicates the pressure in thetreating cylinder throughout the period of 'the pressure treatment. Theinitial pressure required to offset the preliminary air inch so the lineitil starts at this pressure. The pressureincreased `very little in thefirst hour, and had risen only to about 26 pounds, an increase of only 6pounds in one hour, even though approximately `3,333Y pounds ofpreservative liquid had been pumped into the treating cylinder in thisperiod. A-s the increase n pressure o'vi the entire period of one hourWas only 6 pounds to the square inch, relatively little force wasexerted on the timber in 'the charge to compress the timber or vcollapsethe 'wood cells during this period. Hence, it is clear that the'preservative liquid supplied to 'the treating cylinder during thisperiod penetrated into the timber and the slight increase in pressurewhich did occur was that which was necessary to overcome the 'rsistanceto now of liquid through the wood as the depth of penetration of theliquid increased;

ln like manner there `vvas a further gradual increase in pressure duringthe second hour and at the `end of the'se'cond hour the pressure wasabout 33 pounds, 0r an increase of only 7 pounds over the pressurepresent at the end of the first hour. The increase in pressure duringthis periool was so limited in amount, and was distributed over so longa period, that it did not cause collapse of the Wood cells of the timberin the charge, but was sufficient only to overcome resistance of thevvo'od to penetration yof the preservative.

There was va further gradual increase in the pressure during the thirdhour and at the end of the third hour the pressure was approximately 41pounds, or about 8 pounds higher than at the end of the second hour. l

similarly, the pressure gradually increased during the fourth and fifthhours, the pressure at the end of the fourth hour being about 5o pounds,or approximately 9 pounds higher than at the end of the third hour. Atthe end of the fth hour the pressure was about 61 pounds, or about 1lpounds higher than at the end of the fourth hour.

After the fifth hourk the pressure increased rapidly so that at the endof the pressure treating period. the pressure was about 87 pounds, or anincrease of about 26 poundsl in less than an hour. The rapid increase in'the pressure in the treating cylinder near the end of' the pressureperiod indicates the increasing resistance to the penetration of thepreservative into the timber in the cylinder. Ill addition, it showsthat the pelletration of preservative into the timber was appreachingthe point of refusal, and that further increases in pressure, orcontinued exposure of the timber to preservative under pressure, wouldnot materially increase the amount of preservative absorbed bythetimber.

The slow Vgradual increase in pressure which the disgramsshow'to haveoccurred in the treatina cylinder showsthat the liquid supplied to thetreating cylinder'penetrat'ed into the'timber as fast as the liquid wassupplied to the treating cylinder.

The diagram also shows that the rise in pressure in the treatingcylinder was so gradual that the cells in the timber had ampleopportunity to become filled or substantially filled with preservativeliquid so that the pressure within the cells was built up'rapidly enoughto enable the cells to withstand the increased external pressure withthe result that collapse of the cells does not occur. The absence ofcell collapse in timbers treated by this process is confirmed bymicroscopic inspection of pieces of wood treated by this process. On theother hand, when timbers of the same type treated by the conventionalpressure treating processes are examined microscopically, the cell wallsare shown to be collapsed.

It will be observed in Fig. 3 that the slope of the pressure curve issubstantially constant over the major portion of the treating period;inl other words, that the logarithm of the pressure is essentially alinear function of the time over the major portion of the treatingperiod. Thus over the first five hours, 88 per cent of the treatingperiod, the curve is essentially linear. Thereafter as the wood in thetreating cylinder nears saturation the system becomes nearer and nearera pure hydraulic system. The pressure, therefore, begins to increaserapidly thus indicating that the wood has taken up about all thepreservative it will hold. l

It will be observed further that the pressure curve falls below thestraight line defined by points Piti and P2152. As t1 equals zero, thisline may be represented by the equation:

If the preservative is fed to the cylinder ata rate faster than that atwhich the preservative goes into the wood, the system will be shiftedtoward a purely hydraulic system..` Theslope of the log P curve,therefore, increases as afunction of the time as shown between'thefthand sixth hours on the curve. v,

The absence of collapse of the cells in timbers -treated in accordancewith this invention results in uniform deep penetration'of thepreservative liquid. Pieces of wood treated in accordance with thisinvention and then sawed in two at a point some distance from the endsof the piece show that the preservative has thoroughly and uniformlypenetrated the sap wood portion of the timber. Where the pieces Whichhave been treated are round poles, ,the penetration of the Apreservativeliquid is to a substantially uniform depth throughout the circumferenceof the piece.

On the other hand, similar inspection of timbers of the same typetreated yby the conventional pressure treating processes showsv that thepenetration of the preservativeisextremely uneven and non-uniform,beingrelatively, deepbin some areas,and quite shallow in others. Theuniform deep penetration of the preservative secured by this processprovides the maximum protection for the treated timbers with a minimumexpenditure for preservative.

Furthermore, this process does not increase the length of the treatingprocess even though in this process the pressure in the treatingcylinder is much lower throughout a substantial part of the treatingcycle than in conventional processes.

Although this improved process has been illustrated and described inconnection with apparatus which causes preservative liquid to besupplied to the treating cylinder at a constant rate, and thus producesa controlled gradual increase in the pressure in the treating cylinder,it is contemplated that the supply of liquid to the treating cylindermay be regulated in other ways to provide substantially the same gradualincrease in the pressure in the treating cylinder throughout thetreating period.

Similarly, although the process has been illustrated and described inconnection with the treatment of one species of green wood, the processis not limited to use in the treatment of this species of wood, but isequally useful in the treatment of other species of soft woods of lowdensity. Likewise, the process is not limited to use in the treatment ofgreen woods, but is advantageous in the treatment of partially seasonedwoods which are too green to be treated successfully by conventionalprocesses intended for treatment of fully seasoned woods. Accordingly,the term "pine lumber is intended to include all types of soft orconiferous timber and is not limited` to species commonly known aspines.

The specic gravity of the wood is determined by the methods commonlyemployed in the industry. A piece of wood is dried in a kiln or oven ata temperature of C. until it reaches a weight that does not vary. Theweight of the piece after drying is then compared with the weight of avolume of water equal to the volume of the piece of wood in its-greenstate, that is, prior to drying of the piece.

It has been found that when treating green lumber the maximum rate ofsupply of liquid to the treating cylinder which can be safely employedis about 3 pounds per cubic foot of lumber `per hour. As the lumberseasons, it is progressively easier to treat, and on partially seasonedlumber the rate of supply of liquid to the treating cylinder may beincreased to about 4.5 pounds vper cubic foot per hour.`

Although I have illustrated and described one form of improved processfor impregnatingwood with preservative, it -is to be understood that theprocess is not limited to the specific details described and thatnumerous changes and modiications may be made without departing from thespirit and scope of the accompanying claims.

The preferred form of the invention having been thus described, what isclaimed is:

l. In a process for impregnating unseasoned wood in which the wood isfirst dried by heating with steam followed by drawing a vacuum on thethus treated wood and then impregnated with a liquid impregnant themethod .whereby unseasoned pine having a specic gravity less than 0.45may be effectively impregnated which comprises; in the impregnating stepconfining the dried, unseasoned wood and impregnating liquid under iluidpressure ina closed pressure Avessel and pumping additional impregnating:liquid .inist-,he sorlosedivessel at a A substantially constant rateover substantially the entire period of the impregnation, said ratebeing less than 3-4.5 pounds of impregnating liquid per cubic foot ofwood per hour whereby each increment of impregnating liquid pumped intothe lled treating vessel necessarily is forced into the wood and thepressure is a function of the resistance the wood offers to the flow ofimpregnating liquid into its interior.

2. The process of claim 1 in which the treating vessel is completelylled with impregnating liquid and the system is purely a hydraulic oneexcept for the wood in the treating vessel whereby each increment ofimpregnating liquid pumped into the lled treating vessel necessarily isforced into the wood and the pressure is a function of the resistancethe wood oers to the flow of impregnating liquid into its interior.

3. The process of claim 2 in which the pressure does not increase at arate of more than 9 pounds per square inch per hour initially.

4. The process of claim 3, in which the rate of change of pressure overthe rst hour is in the order of 6 pounds per square inch per hour and inwhich the rate of change of pressure does not double itself until afterabout the fourth hour.

5. The process of claim 4 in which the wood treated is green Norway pineand in which the rate specified is about 3 pounds of impregnating liquidper cubic foot of Wood per hour.

6. A process for impregnating unseasoned pine having a specific gravityless than 0.45

10 which comprises charging the wood into a closed pressure vessel,steaming the wood therein, drawing a vacuum on the steamed wood, placingthe wood under a pressure of air in order to charge the Wood with air,displacing the air with impregnating liquid without changing thepressure, and then pumping additional impregnating liquid into thevessel at a substantially constant rate not greater than 3-4.5 pounds ofimpregnating liquid per cubic foot of wood per hour and such that thepressure continually ncreases over the entire period of the impregnationand then releasing the" pressure on the system and withdrawing theimpregnating liquid.

7. The process of claim 6 in which the wood treated is green Norway pineand in which the rate specied is about 3 pounds of impregnating liquidper cubic foot of wood per hour.

MERVIN J. HERMAN.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PROCESS FOR IMPREGNATING UNSEASONED WOOD IN WHICH THE WOOD ISFIRST DRIED BY HEATING WITH STREAM FOLLOWED BY DRAWING A VACUUM ON THETHUS TREATED WOOD AND THEN IMPREGNATED WITH A LIQUID IMPREGNANT THEMETHOD WHEREBY UNSEASONED PINE HAVING A SPECIFIC GRAVITY LESS THAN 0.45MAY BE EFFECTIVELY IMPREGNATED WHICH COMPRISES; IN THE IMPREGNATING STEPCONFINING THE DRIED, UNSEASONED WOOD AND IMPREGNATING LIQUID UNDER FLUIDPRESSURE IN A CLOSED PRESSURE VESSEL AND PUMPING ADDITIONAL IMPREGNATINGLIQUID INTO THE SO-CLOSED VESSEL AT A SUBSTANTIALLY CONSTANT RATE OVERSUBSTANTIALLY THE ENTIRE PERIOD OF THE IMPREGNATION, SAID RATE BEINGLESS THAN 3-4.5 POUNDS OF IMPREGNATING LIQUID PER CUBIC FOOT OF WOOD PERHOUR WHEREBY EACH INCREMENT OF IMPREGNATING LIQUID PUMPED INTO THEFILLED TREATING VESSEL NECESSARILY IS FORCED INTO THE WOOD AND THEPRESSURE IS A FUNCTION OF THE RESISTANCE THE WOOD OFFERS TO THE FLOW OFIMPREGNATING LIQUID INTO ITS INTERIOR.